1. Field of the Invention
The present invention related to a process to form a mold, for the nanoimprint technique, the mold having a pattern for the diffraction grating, a process to form the diffraction grating and a process to form the distributed feedback laser diode (hereafter denoted as DFB-LD) with the diffraction grating.
2. Related Prior Art
Conventionally, the diffraction grating for the DFB-LD has been formed by, after preparing the resist pattern by the interference lithography or the electron beam lithography (EB lithography), etching the semiconductor material with resist pattern as the etching mask. Recently, another technique to form the diffraction grating has been known in which a mold having the grating pattern is pressed against a resin to transfer the grating pattern on this resin, which is often called as the nanoimprint technique. Prior documents, such as the U.S. Pat. No. 7,165,957, and Journal of Vacuum Science and Technology, vol. B14 (6), pp. 4129-4133 (1996), have disclosed the nanoimprint technique.
Because the interference lithography, or the EB lithography, is inevitable to form the patterns with an uniform depth, that is, the aspect ratio of the obtained patterns is uniform within a wafer; accordingly, it is quite hard to modify the depth of the diffraction grating in a limited portion thereof or to control the depth of the resist patterns intentionally so as to compensate the inhomogeneous etching rate within the wafer. Moreover, it is further difficult to grade the duty ratio of the patterns optionally within the wafer. Here, the duty ratio of the pattern means the ratio of the width of the rib against the width of the valley of the pattern. It is also hard or at least necessary for a long time to distribute the pitch of the patterns optionally in the wafer. Thus, the conventional technique of the interference lithography fundamentally limits to the uniform resist patterns within the wafer. That is, only one type of the patterns may be realized in the single wafer by the conventional technique.